Hydrocarbon feedstocks such as aromatic hydrocarbon feedstocks are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). Such feedstocks can be used in a variety of petrochemical processes, such as para-xylene production from an aromatic hydrocarbon feedstock containing benzene, toluene and xylene (BTX), toluene disproportionation, xylene isomerization, alkylation and transalkylation. However, aromatic hydrocarbon feedstocks often contain contaminants comprising bromine-reactive compounds including unsaturated hydrocarbons, such as mono-olefins, multi-olefins and styrenes. These can cause undesirable side reactions in downstream processes. Therefore, these contaminants should be removed from the aromatic hydrocarbon feedstocks before they can be used in other processes.
Improved processes for aromatics production, such as that described in the Handbook of Petroleum Processing, McGraw-Hill, New York 1996, pp. 4.3-4.26, provide increased aromatics yield but also increase the amount of contaminants. For example, the shift from high-pressure semi-regenerative reformers to low-pressure moving bed reformers results in a substantial increase in BI in the reformate streams, which are aromatic hydrocarbon feedstocks for downstream processes. This results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from aromatic hydrocarbon feedstocks, e.g., reformate streams.
Olefins (mono-olefins and multi-olefins) in aromatic hydrocarbon feedstocks are commercially removed by hydrotreating processes. Commercial hydrotreating catalysts have proved active and stable to substantially convert multi-olefins contained therein to oligomers and to partially convert the olefins to alkylaromatics.
The clay treatment of hydrocarbons is widely practiced in the petroleum and petrochemical industries. Clay catalysts are used to remove impurities from hydrocarbon feedstocks in a wide variety of processes. One of the most common reasons for treating these hydrocarbon feedstocks with a clay catalyst system is to remove undesirable olefins, including both multi-olefins and mono-olefins, in order to meet various quality specifications.
More recently, molecular sieves, and particularly zeolites, have been proposed as replacements for clays in the removal of olefinic compounds from aromatic hydrocarbon feedstocks. U.S. Pat. No. 6,368,496 (Brown et al.) discloses a method for removing bromine reactive hydrocarbon contaminants from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active catalyst composition under conditions sufficient to remove mono-olefins. An aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not mono-olefins.
U.S. Pat. No. 6,500,996 (Brown et al.) discloses a method for the removal of hydrocarbon contaminants, such as dienes and olefins, from an aromatics reformate by contacting an aromatics reformate stream with a hydrotreating catalyst and/or a molecular sieve. The hydrotreating catalyst substantially converts all dienes to oligomers and partially converts olefins to alkylaromatics. The molecular sieve converts the olefins to alkylaromatics. The process provides an olefin depleted product which can be passed through a clay treater to substantially convert the remaining olefins to alkylaromatics. The hydrotreating catalyst has a metal component of nickel, cobalt, chromium, vanadium, molybdenum, tungsten, nickel-molybdenum, cobalt-nickel-molybdenum, nickel-tungsten, cobalt-molybdenum or nickel-tungsten-titanium, with a nickel molybdenum/alumina catalyst being preferred. The molecular sieve is an intermediate pore size zeolite, preferably MCM-22. The clay treatment can be carried out with any clay suitable for treating hydrocarbons.
The molecular sieve catalyst has limited lifetimes in hydrocarbon feedstock treatment services partially due to the deposition of hydrocarbons onto the catalyst. The length of service correlates with the amount and the kind of olefinic compounds in the hydrocarbon feedstocks. In general, the molecular sieve catalyst needs to be regenerated after being in hydrocarbon feedstock treatment services for a period of time. The molecular sieve catalyst may be regenerated by burning off the hydrocarbons under oxidative environment, stripping off at least a portion of the hydrocarbons with He, N2, H2, light hydrocarbons, or steam before burning off the remaining hydrocarbons deposition on the catalysts. However, steaming the molecular sieve catalyst under severe conditions may cause irreparable damage to the catalytic activity of the molecular sieve catalyst.
A need exists for an improved process for steaming the molecular sieve catalyst used in the BI reduction. The present invention provides a process for stripping hydrocarbons from a catalyst containing a molecular sieve having a structure type of MWW using steam under steaming conditions such that to regenerate at least a portion of its catalytic activity for the BI reduction application.